Field of the Disclosure
The present disclosure relates to a switching power supply, and more particularly, to a controller and a controlling method of a switching power supply which reduces EMI (Electromagnetic Interference).
Description of the Related Art
A switching power supply is usually an EMI source due to fast switching of power transistors in operation. It is a common concern in industry to reduce EMI as required by some standards of the power supply. A conventional approach to reduce EMI of the switching power supply is to add an EMI filter into the circuit, which, however, increases cost and size of the power supply, deteriorates high-frequency performance, and increases power consumption.
The frequency jittering control is an effective method for suppressing conducted EMI, and has been widely used in some low-power switching power supplies. The method changes an operating frequency of the switching power supply periodically or non-periodically in a range, so that harmonic interference energy is dispersed and the EMI standards are met.
However, the conventional frequency jittering control in the switching power supply is based on the principle of changing a system switching frequency of the switching power supply. Although it can suppress conducted EMI, the power transistor will not be turned on at idea moment due to variation of the system switching frequency. The frequency jittering control leads to higher conduction loss, and lower system efficiency and higher heat dissipation. Moreover, because the power transistor is turned on and off at a high voltage, additional electromagnetic radiation may occur, which results in degradation of EMI.
Thus, it is desirable to further improve the frequency jittering control in a switching power supply to reduce conduction loss and suppress electromagnetic radiation of the power transistor.